Synchronized Diagnostic Measurement for Cochlear Implants

ABSTRACT

Objective measurement of cochlear implant operation is described which coordinates the delivery to a patient of an acoustic signal and an electrical signal including coordinating a delay time between the acoustic signal and the electrical signal. The acoustic signal is developed as an acoustic stimulation input to the ear canal of a patient, and the electrical signal is developed as an electrical stimulation input to intracochlear electrodes of a cochlear implant. The evoked response in the patient to the delivered signals is then measured including measuring a far field response associated with the skin surface of the patient.

This application is a divisional of U.S. patent application Ser. No.12/417,041, filed Apr. 2, 2009, which in turn claims priority from U.S.Provisional Patent Application 61/042,054, filed Apr. 3, 2008, both ofwhich are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to medical implants, and more specificallyto diagnostic measurement in cochlear implant systems.

BACKGROUND ART

Cochlear implants (CI) help profoundly deaf or severely hearing impairedpersons to perceive environmental sounds. Unlike conventional hearingaids which just apply an amplified and modified sound signal, a cochlearimplant is based on direct electrical stimulation of the auditory nerveso that hearing impressions most similar to normal hearing are obtained.

A cochlear implant system consists of two main parts, an external speechprocessor and the implanted stimulator. The speech processor contains apower supply and is used to perform signal processing of an acousticinput signal to extract stimulation parameters for the implantedstimulator. The implanted stimulator generates stimulation patterns anddelivers them to auditory nervous tissue by an electrode array whichusually is positioned in the scala tympani in the cochlea. A wirelessconnection between the speech processor and the implanted stimulator canbe established by encoding digital information in an rf-channel andcoupling the signal percutaneously using an inductive coupled coilsarrangement. The implanted stimulator decodes the information byenvelope detection of the rf signal.

Stimulation strategies employing high-rate pulsatile stimuli inmulti-channel electrode arrays have proven to be successful in givinghigh levels of speech recognition. One example is the ContinuousInterleaved Sampling (CIS) strategy, as described by Wilson et al.,Better Speech Recognition With Cochlear Implants, Nature, vol.352:236-238 (1991), which is incorporated herein by reference. For CIS,symmetrical biphasic current pulses are used which are strictlynon-overlapping in time. The rate per channel typically is higher than800 pulses/sec. Other stimulation strategies may be based onsimultaneous activation of electrode currents.

For high-rate pulsatile stimulation strategies, some patient specificparameters typically need to be determined. This is done some weeksafter surgery in a fitting procedure. For given phase duration ofstimulation pulses and for a given stimulation rate, two key parametersto be determined for each stimulation channel include:

-   -   1. the minimum amplitude of biphasic current pulses necessary to        elicit a hearing sensation (Threshold Level, or THL); and    -   2. the amplitude resulting in a hearing sensation at a        comfortable level (Most Comfortable Level, or MCL).        For stimulation, only amplitudes between MCL and THL for each        channel are used. The dynamic range between MCL and THL        typically is between 6-12 dB. However, the absolute positions of        MCLs and THLs vary considerably between patients, and        differences can reach up to 40 dB. To cover these absolute        variations, the overall dynamic range for stimulation in        currently used implants is typically about 60 dB.

There are several methods of setting the MCLs and THLs. For example,they can be estimated during the fitting procedure by applyingstimulation pulses and asking the patient about his/her subjectiveimpression. This method usually works without problems withpostlingually deaf patients. However, problems occur with prelinguallyor congenitally deaf patients, and in this group, all ages—from smallchildren to adults—are concerned. These patients are usually neitherable to interpret nor to describe hearing impressions, and only roughestimations of MCLs and THLs are possible based on behavioral methods.Especially the situation of congenitally deaf small children needs to bementioned here. An adequate acoustic input is especially important forthe infant's speech and hearing development, and this input in manycases can be provided with a properly fitted cochlear implant.

One approach for an objective measurement of MCLs and THLs is based onthe measurement of the EAPs (Electrically Evoked Action Potentials), asdescribed by Gantz et al., Intraoperative Measures of ElectricallyEvoked Auditory Nerve Compound Action Potentials, American Journal ofOtology 15 (2):137-144 (1994), which is incorporated herein byreference. In this approach, the recording electrode is usually placedat the scala timpani of the inner ear. The overall response of theauditory nerve to an electrical stimulus is measured very close to theposition of the nerve excitation. This neural response is caused by thesuper-position of single neural responses at the outside of the axonmembranes. The amplitude of the EAP at the measurement position isbetween 10 μV and 1800 μV. Information about MCL and THL at a particularelectrode position can first of all be expected from the so called“amplitude growth function,” as described by Brown et al., ElectricallyEvoked Whole Nerve Action Potentials In Ineraid Cochlear Implant Users:Responses To Different Stimulating Electrode Configurations AndComparison To Psychophysical Responses, Journal of Speech and HearingResearch, vol. 39:453-467 (June 1996), which is incorporated herein byreference. This function is the relation between the amplitude of thestimulation pulse and the peak-to-peak voltage of the EAP. Anotherinteresting relation is the so called “recovery function” in whichstimulation is achieved with two pulses with varying interpulseintervals. The recovery function as the relation of the amplitude of thesecond EAP and the interpulse interval allows conclusions to be drawnabout the refractory properties and particular properties concerning thetime resolution of the auditory nerve.

Besides cochlear implant systems as such, some subjects with someresidual hearing (partial deafness) are now benefiting from hybridsystems such as combined electric and acoustic stimulation (EAS) as wasfirst described in von Ilberg et al., Electric-Acoustic Stimulation OfThe Auditory System, ORL 61:334-340 (1999), which is incorporated hereinby reference. EAS systems combine the use of a conventional hearing aid(HA) device to provide acoustic-mechanical stimulation of lower audiofrequencies to the subject's ear drum and a cochlear implant (CI) toprovide intracochlear electrical stimulation of higher audio frequenciesto the auditory nerve. For example, see Lorens et al., Outcomes OfTreatment Of Partial Deafness With Cochlear Implantation: A DUET Study,Laryngoscope, 2008 February: 118(2):288-94, which is incorporated hereinby reference.

SUMMARY OF THE INVENTION

Embodiments of the present invention are directed to objectivediagnostic measurement for cochlear implant patients utilizingsynchronized acoustic and electrical signals including coordinating adelay time between the acoustic signal and the electrical signal. Theacoustic signal is developed as an acoustic stimulation input to the earcanal of a patient, and the electrical signal is developed as anelectrical stimulation input to intracochlear electrodes of a cochlearimplant. The evoked response in the patient to the delivered signals isthen measured including measuring a far field response associated withthe skin surface of the patient.

In further specific embodiments, the measured response may be analyzedto diagnose auditory perception of the patient. For example, this mayinclude determining a frequency-specific response relative to specificposition of the electrodes in the cochlea. An embodiment may be used tocustomize operation of the cochlear implant for the patient based on themeasured response.

In a specific embodiment, measuring the evoked response may includemeasuring a near field response in associated tissue near to theintracochlear electrodes.

In some embodiments, the cochlear implant may be a bilateral cochlearimplant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows various functional blocks in one specific embodiment of thepresent invention.

FIG. 2 shows an example of the measured evoked response representing anear field recording from the synchronized electrical and acousticstimulation inputs.

FIG. 3 shows an example of the measured evoked response representing afar field recording from the synchronized electrical and acousticstimulation inputs in a first patient.

FIG. 4 shows an example of the measured evoked response representing afar field recording from the synchronized electrical and acousticstimulation inputs in a second patient.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

Embodiments of the present invention are directed to an objectivemeasurement system for a cochlear implant which coordinates andsynchronizes an acoustic stimulus and an electrical stimulus ofremaining hair cells and neural cells. The resulting evoked response isrecorded and analyzed with near and/or far field measurements. Thisarrangement is especially useful in diagnostics of patients implantedwith a cochlear implant (e.g., partial deafness), helping to optimizethe fit for patients of their speech processor and the cochlear implantstimulation. These measurements can also be useful for identifyingproperties of the auditory nerve and higher levels of the auditorypathway and for acquiring information regarding the preservation of theremaining hearing in a patient. The combination of the near field andfar field recordings may specifically be useful for identifyingfrequency specific placement of the stimulation electrodes within thecochlea. A system may also be useful for research purposes such as intothe properties of frequency and/or time dependent stimuli, informationabout the movement of the basilar membrane, obtaining importantindications as to whether electrical stimulation directly stimulates theremaining hair cells or whether it stimulates cells of the auditorynerve or for developing of new speech coding strategies for cochlearimplants.

FIG. 1 shows one example of a specific objective measurement system inwhich an acoustic stimulation input 102 develops an acoustic signal inthe ear canal of a patient 103. An electrical stimulation input 101 alsodevelops an electrical signal for intracochlear electrodes 110 in acochlear implant 109 in the patient 103. Typically, a percutaneousinductive coil arrangement 108 would be used to couple the electricalsignal from the electrical stimulation input 101 across the skin of thepatient 103 into the cochlear implant 109. FIG. 1 shows a unilateralcochlear implant 109 on just one side of the patient 103, but otherembodiments may be based on a bilateral system of cochlear implants onboth the left and right sides of the patient 103. A control module 104coordinates the delivery of the acoustic signal and the electricalsignal by the acoustic stimulation input 102 and electrical stimulationinput 101 respectively. For example, the control module 104 maycoordinate the signal delivery by coordinating a delay time between theacoustic signal and the electrical signal. A measurement module 106measures the evoked response in the patient 103 to the deliveredsignals. In specific embodiments, these elements may be implemented asdedicated hardware devices, or computer software running on generic orspecific computer devices, or some combination of hardware and software.

In the specific embodiment shown in FIG. 1, the measurement module 106is within and a part of the control module 104. For example, themeasurement module 106 may be a software routine which forms a part of alarger software application that constitutes the control module 104. Inother embodiments, the measurement module 106 and the control module 104may be separate and independent, and indeed, may run on differentcomputers. Similarly, in some embodiments, the electrical stimulationinput 101 and/or the acoustic stimulation input 102 may be developed byor delivered by their own associated computers which may or may not beseparate and independent of the control module 104 and any otherassociated computer.

The system shown in FIG. 1 also includes a diagnosis module 105 foranalyzing the measured response to diagnose the auditory perception ofthe patient. For example, the diagnosis module 105 may analyze themeasured response by determining a frequency-specific response relativeto the specific position of the electrodes in the cochlea. An embodimentmay also include a fitting module for customizing operation of thecochlear implant for the patient 103 based on the response developed bythe measurement module 106. Such a fitting module may be a separatedevice or software module, or may form a portion of one of the othersystem elements such as the measurement module 106 or the diagnosismodule 105.

To develop the evoked response measurement, the cochlear implant 109 andthe intracochlear electrodes 110 may include one or more near fieldmeasurement sensors for measuring a near field response in associatedtissue near to the intracochlear electrodes 110. Also shown are one ormore far field measurement sensors 107 for measuring a far fieldresponse associated with the skin surface of the patient.

Using the coordinated and synchronized electrical stimulation input 101and acoustic stimulation input 102 several types of evoked potentialscan be recorded. Near field recordings (i.e. from the implantedintracochlear electrodes 110) and/or far field recordings, e.g., fromthe far field sensing electrodes 107 placed on the head according to thespecific types of the auditory evoked potentials measurements. Possiblemeasurements may include short latency responses such as compound actionpotentials and auditory brainstem responses, middle late potentials suchas middle late responses, and late cortical responses resulting from theelectric and acoustic stimulation.

FIG. 2 shows an example of the measured evoked response of a near fieldrecording resulting from the synchronized electrical and acousticstimulation inputs. Specifically, FIG. 2 shows measurement ofacoustically and electrically evoked compound action potential near theintracochlear electrodes 110 from the synchronized electricalstimulation input 101 and acoustic stimulation input 102 at the mostcomfortable level for the patient 103. FIG. 3 shows an example of farfield recordings from a first patient at the most comfortable level ofacoustic and electrical stimuli. Trace A shows a set of far fieldrecordings for electrical stimulation only, Trace B shows an examplewith both acoustic and electrical stimulation, and Trace C showsacoustic stimulation only. FIG. 4 shows a similar set of traces for asecond patient where Trace A shows recordings obtained from electricaland acoustic stimulation at the most comfortable level, Trace B showselectrical stimulation at the most comfortable level and acousticstimulation at 10 dB lover level than the patient's most comfortablelevel, and Trace C shows acoustic stimulation at the most comfortablelevel.

Embodiments of the present invention may be especially useful for EASpatients because of their better preserved hearing postoperatively. Forexample, EAS patients as a group demonstrate significantly higherpost-operative speech score results than regular CI patients. In EASpatients, the optimal cochlear implant (CI) parameters as determinedfrom the measurements obtained by embodiments of the present inventionmay in turn influence hearing aid (HA) parameters, e.g., the frequencyranges of each. And rather than just an implanted speech processor forthe CI, EAS systems may also be based on an overall system speechprocessor which coordinates the operation of both the cochlear implant(CI) and the acoustic-mechanical hearing aid (HA).

Embodiments of the invention may be implemented in any conventionalcomputer programming language. For example, preferred embodiments may beimplemented in a procedural programming language (e.g., “C”) or anobject oriented programming language (e.g., “C++”, Python). Alternativeembodiments of the invention may be implemented as pre-programmedhardware elements, other related components, or as a combination ofhardware and software components.

Embodiments can be implemented as a computer program product for usewith a computer system. Such implementation may include a series ofcomputer instructions fixed either on a tangible medium, such as acomputer readable medium (e.g., a diskette, CD-ROM, ROM, or fixed disk)or transmittable to a computer system, via a modem or other interfacedevice, such as a communications adapter connected to a network over amedium. The medium may be either a tangible medium (e.g., optical oranalog communications lines) or a medium implemented with wirelesstechniques (e.g., microwave, infrared or other transmission techniques).The series of computer instructions embodies all or part of thefunctionality previously described herein with respect to the system.Those skilled in the art should appreciate that such computerinstructions can be written in a number of programming languages for usewith many computer architectures or operating systems. Furthermore, suchinstructions may be stored in any memory device, such as semiconductor,magnetic, optical or other memory devices, and may be transmitted usingany communications technology, such as optical, infrared, microwave, orother transmission technologies. It is expected that such a computerprogram product may be distributed as a removable medium withaccompanying printed or electronic documentation (e.g., shrink wrappedsoftware), preloaded with a computer system (e.g., on system ROM orfixed disk), or distributed from a server or electronic bulletin boardover the network (e.g., the Internet or World Wide Web). Of course, someembodiments of the invention may be implemented as a combination of bothsoftware (e.g., a computer program product) and hardware. Still otherembodiments of the invention are implemented as entirely hardware, orentirely software (e.g., a computer program product).

Although various exemplary embodiments of the invention have beendisclosed, it should be apparent to those skilled in the art thatvarious changes and modifications can be made which will achieve some ofthe advantages of the invention without departing from the true scope ofthe invention.

1. A method for objective diagnostic measurement for cochlear implantsubjects, the method comprising: in an audio diagnostic measurementsystem: coordinating the delivery to a patient of an acoustic signal andan electrical signal so that: i. the acoustic signal is developed as anacoustic stimulation input to the ear canal of a patient, and ii. theelectrical signal is developed as an electrical stimulation input tointracochlear electrodes of a cochlear implant, wherein coordinating thedelivery includes coordinating a delay time between the acoustic signaland the electrical signal; and measuring an evoked response in thepatient to the delivered signals, wherein the evoked response includes afar field response associated with the skin surface of the patient.
 2. Amethod according to claim 1, further comprising: analyzing the measuredresponse to diagnose auditory perception of the patient.
 3. A methodaccording to claim 2, wherein diagnosing the auditory perceptionincludes determining a frequency-specific response relative to specificposition of the electrodes in the cochlea.
 4. A method according toclaim 1, further comprising: customizing operation of the cochlearimplant for the patient based on the measured response.
 5. A methodaccording to claim 1, wherein measuring the evoked response includesmeasuring a near field response in associated tissue near theintracochlear electrodes.
 6. A method according to claim 1, wherein thecochlear implant is a bilateral cochlear implant.
 7. An objectivemeasurement system for a cochlear implant, the system comprising: anacoustic stimulation input for developing an acoustic signal to the earcanal of a patient; an electrical stimulation input for developing anelectrical signal for intracochlear electrodes of a cochlear implant; acontrol module for coordinating the delivery of the acoustic signal andthe stimulation signal including coordinating a delay time between theacoustic signal and the electrical signal; and a measurement module formeasuring the evoked response in the patient to the delivered signals,including a far field measurement sensor for measuring a far fieldresponse associated with the skin surface of the patient.
 8. A systemaccording to claim 7, further comprising: a diagnosis module foranalyzing the measured response to diagnose auditory perception of thepatient.
 9. A system according to claim 8, wherein the diagnosis moduleanalyzes the measured response by determining a frequency-specificresponse relative to specific position of the electrodes in the cochlea.10. A system according to claim 7, further comprising: a fitting modulefor customizing operation of the cochlear implant for the patient basedon the measured response.
 11. A system according to claim 7, wherein themeasurement module includes a near field measurement sensor formeasuring a near field response in associated tissue near theintracochlear electrodes.
 12. A system according to claim 7, wherein thecochlear implant is a bilateral cochlear implant.